I suspect that the ease of gaining an advantage is just a way to reduce rng. The current system of gaining an advantage such as attack from behind or high terrain is quite easy for the average player to understand.
Due to the fact that Larian targets a fairly wide range of players, RNG reduction makes sense, although you can argue whether it is the best solution (not too much in my opinion).
A flat bonus does more to reduce rng than Advantage. Especially for ability checks/saving throws where a 1 isn't an automatic failure. And even more especially since Larian's rng has streaks, where a 5 for the first d20 makes it more likely that the second d20 will also be a 5.
To bring the discussion back to proficiency bonus, modifying this value for various difficulty levels would be a much a much better option than all the workarounds Larian has implemented to improve success chance (backstab, height advantage, inspiration, loaded dice).
